In comparison to aqueous electrodes in the related art such as a Ni—Cd electrode or a Ni—H electrode, non-aqueous lithium ion electrodes have a high energy density and can be easily reduced in size, and therefore non-aqueous lithium ion electrodes are widely used for mobile devices such as mobile phones, mobile information terminals, and personal computers. As a positive electrode material for non-aqueous lithium ion batteries, currently, LiCoO2 has been put into practical use and is commonly used.
However, when LiCoO2 is used as it is as a positive electrode material for non-aqueous lithium ion batteries in the field of large-sized batteries or the like that are mounted on hybrid vehicles, electric vehicles, and uninterruptible power systems which are expected to be developed in the future, a variety of problems may occur as described below.
One of those problems is the difficulties in the resource and cost aspects in stably procuring a large amount of cobalt (Co) since LiCoO2 includes cobalt (Co), which is a rare metal.
In addition, since LiCoO2 generates oxygen at a high temperature, safety should be sufficiently considered in preparation for the cases of abnormal heat generation or short-circuiting in a battery, and therefore it is highly risky to apply LiCoO2 to large-sized batteries without sufficiently considering safety.
As a result, in recent years, instead of a positive electrode active material using LiCoO2, positive electrode materials which are cheap and safe, and have a phosphate backbone have been suggested. Among them, LiFePO4 having an olivine structure is gaining attention as a positive electrode material which is safe and also has no problem in the resource and cost aspects, and is being studied and developed across the world (For example, see Patent document 1, Non-Patent document 1, or the like).
Olivine-type positive electrode materials represented by LiFePO4 contain iron (Fe). In comparison to cobalt and manganese, iron is a rich natural resource and thus is cheap. In addition, due to the covalent bonding between phosphorous and oxygen, the olivine structure does not emit oxygen at a high temperature unlike cobalt-based positive electrode active materials such as LiCoO2, and is also a material excellent in terms of stability.
However, even for LiFePO4 having the above advantages, problems have been pointed out in the characteristics aspect.
One problem is a low conductivity thereof, but in relation to which a number of attempts to improve the conductivity have been carried out in recent years by, particularly, forming the complex of LiFePO4 and carbon, coating carbon on the surface of LiFePO4, or the like.
Another problem is a low degree of diffusion of lithium ions during charging and discharging. For example, in compounds having a layered structure such as LiCoO2, or a spinel structure such as LiMnO2, lithium ions diffuse in two dimension or three dimension during charging and discharging; however, in compounds having an olivine structure such as LiFePO4, lithium diffuses only in one dimension. Furthermore, the electrode reaction during charging and discharging is a two-phase reaction in which LiFePO4 and FePO4 are repeatedly converted, and therefore LiFePO4 is considered to be disadvantageous for high-speed charging and discharging.
As methods for solving the above problems, the method which is considered to be most effective is the reduction of the diameters of LiFePO4 particles. That is, even when lithium ions diffuse in one dimension, if diffusion distance is shortened by the reduction of particle diameters, it is considered that an increase in the speed of charging and discharging can be coped with.
In the past, as a method of synthesizing LiFePO4, the solid-phase method was used. In the solid-phase method, since raw materials of LiFePO4 are mixed in a stoichiometric proportion and then are fired in an inert atmosphere, there are problems in that LiFePO4 having a desired composition cannot be obtained without the skilled selection of firing conditions, and it is difficult to control particle diameters such that it is difficult to reduce particle diameters. Here, as a method of reducing the particle diameters of LiFePO4, studies regarding a liquid-phase synthesis method in which a hydrothermal reaction is used are underway.
The advantage of the hydrothermal reaction is that highly pure products can be obtained at a markedly low temperature in comparison to a solid-phase reaction. However, in the hydrothermal reaction, the control of particle diameters is strongly dependent on factors related to reaction conditions such as a reaction temperature or time. In addition, when the control is performed using the above factors, the performance of a manufacturing apparatus strongly affects the control, which causes difficulties in reproducibility.
Therefore, as a method of generating the fine particles of LiFePO4 by the hydrothermal reaction, a method is suggested in which organic acid or ion such as CH3COO−, SO42− or Cl− is also fed to a solvent, or Li is excessively added during the hydrothermal reaction, thereby obtaining the fine particles of single-phase LiFePO4 (for example, see Patent document 2 and Non-patent document 2). In addition, a method is suggested in which reaction intermediates are mechanically crushed so as to obtain the fine particles of LiFePO4 having small particle diameters (Patent document 3).
However, discharge voltage of LiFePO4 is low, and thus is considered as an electrode material not suitable for uses in which a high output is required, for example, electric tools and hybrid vehicles. As a result, as positive electrode materials having the olivine structure other than LiFePO4, LiMnPO4, LiCoPO4 or the like, which are positive electrode materials that can use the stability of the olivine structure and can be used at a high voltage, are considered as alternatives.
Patent Documents
Patent document 1: Japanese Patent No. 3484003
Patent document 2: Japanese Unexamined Patent Application, First Publication No. 2008-66019
Patent document 3: Japanese Unexamined Patent Application Publication (Translation of PCT Application) No.-2007-511458
Non Patent Document
Non-Patent document 1: “Phospho-olivine as Positive-Electrode Material for Rechargeable Lithium Batteries,” by A. K. Padhi et al., P 1188, Issue 4, Vol. 144, J. Electrochem. Soc. (1997)
Non-Patent document 2: “Synthesis of LiFePO4 cathode active material for Rechargeable Lithium Batteries by Hydrothermal Reaction,” by Keisuke Shiraishi, Young Ho Rho and Kiyoshi Kanamura, P. S58 to S62, Issue 1305, Suppl. 112, J. Ceram. Soc. Jpn (2004).